test(pandas): re-enable `test_udaf_window_nan` test case

[kszucs]

What happened?

Follow-up for https://github.com/ibis-project/ibis/pull/7797

ibis/backends/pandas/tests/test_udf.py::test_udaf_window_nan is failing for pandas, but duckdb produces the same result while the expected is different. Need to check whether the expected value is correct or not and fix the implementations if required.

What version of ibis are you using?

the-epic-split

What backend(s) are you using, if any?

No response

Relevant log output

No response

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